Merging cells in header do not update heading rows properly

[jodator]

📝 Provide detailed reproduction steps (if any)

1. Open "coffee table" sample.
2. Merge cells "milk" and two below.

✔️ Expected result

The table features works properly (merging cells in body, setting heading rows, etc).

❌ Actual result

Table has one heading row (visually) but model have heading rows = 2.

📃 Other details

I don't know which command should take care of this (`TableUtils.removeRow()`, commands or a post-fixer).

Similar thing happen on v18.0.0 using merge table cell left/right/up/down buttons:

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[jodator]

Relates to: #6630.